Background/Purpose: The etiology and pathogenesis of systemic sclerosis (SSc) are poorly understood; however, an increasing body of evidence supports an early inflammatory phase that precedes fibrosis. Circulating monocytes likely play a critical role in SSc progression through secretion of pro-inflammatory molecules and as precursors of macrophages that can reorganize the extracellular matrix (ECM) to leading to the development of end-organ fibrosis. Here we evaluate the transcriptional similarities between circulating classical monocytes and macrophages present in the skin of SSc patients.

Methods:  Classical monocytes (CMo) were sorted using multiparameter fluorescence-activated cell sorting (FACS) from early diffuse cutaneous (dc) SSc sample blood obtained through the Prospective Registry of Early Systemic Sclerosis (PRESS) cohort. Bulk RNA-seq was performed, and transcriptional profiles were analyzed along with age-, sex-, and ethnicity-matched controls.  Additionally, CD45⁺ immune cells, as well as CD31⁺ endothelial cells were FACS-sorted from skin biopsies of one SSc patient and one control patient and prepared for single-cell RNA-seq.

Results: There was an expansion in the skin macrophages from 36 cells in the control sample to 67 cells in the patient sample but no significant difference in the quantity (as a percent of total CD45+ cells) of circulating CMo (p=0.134) between controls and SSc patients. The differentially expressed genes in the circulating CMo were identified using DESeq2. Of the 152 significantly up-regulated genes (DESeq2, p< 0.05, Log2 Fold change in expression >1) observed in the circulating CMo population and the 290 up-regulated genes (Log2 Fold change in expression >1) found in the skin macrophage cluster compared to their respective controls, we find 23 genes in common (p< 1.23×10-8, hypergeometric distribution test). These shared genes are involved in processes that include ‘inflammatory response’ (p< 6.56×10-4, IL10, IL8, IL1B, CXCR4), ‘regulation of mononuclear cell proliferation’ (p< 7.65×10-5, IL1B, IL10, MNDA, PNP), ‘dendritic cell migration’ (p< 5.07×10-4, GPR183, CXCR4), ‘cytokine-mediated signaling pathway’ (p< 1.68×10-5, IL10, VEGF, CXCR4, IFI6), ‘negative regulation of epithelial cell proliferation’ (p< 4.45×10-4, RGCC, THBS1, EREG), ‘regulation of endothelial cell proliferation’ (p< 4.02×10-5, VEGFA, IL10, RGCC, THBS1).

Conclusion: These data indicate that a common transcriptional signature exists between circulating classical monocytes and macrophages present in the skin of SSc patients, potentially suggesting that macrophage-specific pathways that have gone awry at the site of fibrosis can be detected in a circulating precursor population. Future studies will focus on interrogating the penetrance of this gene signature by cross-referencing blood monocyte and skin macrophage transcriptional profiles from the same patient to determine whether this signature in circulating classical monocytes correlates with the severity of skin fibrosis and/or serves as a predictive marker of disease.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/a-common-transcriptional-signature-is-present-in-circulating-classical-monocytes-and-skin-macrophages-in-systemic-sclerosis/